On Hit: Slows Enemy Target (Duration and severity of effect are determined by range, at a longer distance the enemy will be slowed a greater amount and for longer)
On Headshot: Stuns Enemy Target (Duration is determined by range, at a longer distance the enemy will be stunned for longer)
Weapon makes minimal sound when fired
-75% Damage Penalty (10 Base Damage)
-83% Reduced Clip Size (1 Shot)

This weapon would give the Spy a more supportive role, more comfortably placing him in the "Support" category, as well as giving a powerful tool that cannot be easily abused, but rewards skillful play and good aim. This weapon would not be ideal for personal defense, as the pitiful damage can only get you so far, and if you're in a dangerous situation, you're probably too close to the enemy for the effects to be very useful.

Let me know what you think.